1. Field of the Invention
This invention relates generally to the art of railway track circuits. More particularly, the invention relates to a single-element coded AC railway track circuit which is easily adaptable to a double-element, phase-selective configuration. A single-element track circuit uses a receiver requiring only one input signal which is transmitted to it via the track from a distant source. A double-element track circuit uses a receiver requiring such a signal plus an additional input signal which is supplied locally--i.e., at the receiver location.
2. Description of the Prior Art
A railway is a mode of transportation necessarily limited to "one degree of freedom." That is to say, a railway vehicle can only travel back and forth along a track. It cannot alter its path to avoid other traffic. In order to prevent vehicles on the same track from overtaking one another, a block signalling scheme has been devised whereby the track is divided into segments, or "blocks," of a length greater than the stopping distance of a train. Normally, only one train is allowed in a particular block at a time. Typically, wayside indicators placed before an upcoming block indicate to the locomotive operator whether or not an upcoming block is occupied. If so, the operator will know to adjust the speed of his train to avoid a problem.
Railway signalling has been traditionally controlled by the track circuit. The track circuit is essentially an electrical circuit in which the rails within a block provide electrical connection between an electrical signal transmitter and an electrical signal receiver. Electrical separation between adjacent blocks may be provided by insulating joints. The transmitter impresses an electrical signal into the rails at the transmit end of the block which may be received by the receiver at the opposite end if the block is unoccupied and no state of broken rail exists. The receiver, such as a relay, can then operate to display an appropriate aspect on the wayside indicator.
The original track circuits operated only on direct current. It was soon discovered, however, that alternating current track circuits were less susceptible to stray direct currents which could enter the system, for example, through the ground. One early AC track circuit was known as the "Universal Code" track circuit. This single-element circuit operated on a coded AC track signal using a resonant unit tuned to the AC frequency. The track signal was then rectified to operate a code-following DC relay. The "Universal Code" track circuit, however, had one notable disadvantage. Specifically, AC track circuits using insulating joints to provide electrical separation between blocks are inherently more likely than DC track circuits to disrupt operation of wayside indicators in adjacent blocks if the insulating joints break down. This is because AC relays cannot discriminate between polarities as some DC relays can. Staggered rail polarities in adjacent blocks can be used to provide broken down joint protection with DC track circuits, but not with AC track circuits having polarity-insensitive AC relays.
The problem of insulated joint breakdown is especially acute in electric-train territory, also called electrified railway territory, where the two rails also carry propulsion return currents. Here, adjacent blocks are connected by impedance bonds which, by autotransformer action, can allow the full track signal voltage at the transmit end of an adjoining block to feed across a single defective joint into the receiver end of the other block. The Universal Code track circuit employed a complicated "non-vital" lock-out circuit for broken down joint protection. The term "non-vital" signifies that it was theoretically possible for the lock-out circuit to fail in such a way that an unoccupied indication could be given for an occupied block under broken down joint conditions.
In order to combat this shortcoming, a double-element Phase-Selective Track Circuit was developed. With this coded track circuit, adjacent blocks are fed by a common AC energy source, but with the rails having opposite relative polarity. A Phase-Selective Unit placed between the rails and the receiver relay receives a local input from the same AC source. The Phase Selective Unit contains circuitry which distinguishes the desired track signal from that of adjacent blocks based on their phase relationship with respect to the local input. The Phase-Selective Unit operates the receiver relay only when a track signal of the proper polarity compared to the local signal is received. Thus, if an insulating joint separating the blocks breaks down and current from an adjacent block enters the Phase-Selective Unit, this current will be ineffective to operate the relay. For phase referencing, the same AC energy source is required to supply both the track signal and the local signal; therefore, a feed voltage line running the entire length of the respective track circuit is required.
Block lengths in such double-element track circuits may typically be up to 6,000 feet or more. Further, many track circuits can be cascaded. Thus, the additional cost of the source line can be significant. Also, in some locations the theft of copper wire has been a problem which has caused frequent track circuit outages, as well as expensive repairs.